Stereophonic sound reproduction system



Oct. 13, 1964 c. 5. BROWN 3,153,120

STEREOPHONIC scum: REPRODUCTION SYSTEM Filed June 5, 1961 1 1! LEFT CHANNEL 26 RIGHT CHANNEL AMPLIFIER .2 L, AMPLIFIER i4 (6: H 222 a 15 1a 12 2 13 2 27 a. M 28 28A 5v. L 5 av. JV-

4 .58 a? 39 8 33 -55 as j a? Y ALL. SPEAKERS SIDE SPEAKERS SIDE SPEAKER T'WL'ETER IOO CYCLES IOOO lgiqg INVENTOR.

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United States Patent 3,153,120 STEREQPHONIC SQUND REPRODUCTION SYSTEM Charles Scott Brown, Grand Rapids, Mich, assignor to Automatic Canteen Company of America, Chicago, Ill., a corporation of Delaware Filed June 5, 1961, Ser. No. 114,778 5 Claims. ((11. 179---1) The present invention relates to stereophonic reproduction of sound and has, as one of its principal objectives, the provision of means for obtaining a lifelike stereophonic effect Without the necessity of utilizing speakers spaced a substantial distance from each other.

In this connection it may be explained that stereophonic reproduction of sound has heretofore ordinarily required the use of at least two speakers, quite widely separated from each other, and that prior attempts to place the speakers sufficiently close together to be conveniently housed in a single cabinet has usually resulted in reducing the effective listening area almost to the vanishing point, or in an effect which is essentially monaural sound with much of the lifelike nature and liveness gone.

It is accordingly among the objects of the present invention to provide a method and apparatus for stereophonic reproduction of sound capable of successful operation with the entire unit, and all speakers connected therewith, housed within a single relatively compact cabinet.

A further object of the invention is to provide a stereophonic reproduction system so designed as to avoid the overly localized listening area characteristic of many stereophonic systems, and to provide for uniform stereophon ie effect throughout a relatively wide area. A related object is to accomplish the stereophonic effect uniformly across the entire audio spectrum covered by the speakers, and by apparatus not dependent upon nor overly infiuenced by size, shape, proportions or acoustic qualities of the room in which the unit is housed.

A still further object of the invention is to accomplish a more lifelike stereophonic effect than heretofore available withmost types of equipment, and specifically to avoid the so-called hole in the middle effect, wherein sounds coming from the far left or far right of the recording field are clearly heard, but wherein sounds originating in the middle area are unclear, unrealistic, or give the impression of emerging from some point behind the speakers.

A further and quite important object is to provide apparatusand method capable of achieving the effects noted above,'yet compatible with present conventional stereo equipment; wherein effects, according to the present invention, may be obtained by conventional double-channel amplifying systems, energized by the signals from ordinary double-channel records ortapes. Another object is to provide a stereo system having the features indicated heretofore, yet compatible with monaural records, so that satisfactory reproduction of such records is accomplished without mechanical or circuit changes.

The present inventor has developed means whereby the above objects may be successfully accomplished in a reproducing system compact enough to be housed in a single cabinet, of size comparable to conventional home phonographs or jukeboxes. The accomplishment of the purpose involves the coaction of several pieces of equipment, including a pair of speakers placed in close proximity to each other but oppositely directed and in opposed phase relation to each other, with means for energizing each of these speakers by a composite signal made up of information drawn from each of the two signal channels conventional in a double-channel stereo record or tape,

7 3,153,120 Patented Oct. 13, 1964 for example, and with a forwardly directed central speaker to which some signal information from each channel is supplied in a manner such that it will operate at a volume level much below the side speakers.

A present preferred embodiment of the invention is schematically illustrated in the drawings attached to and forming a part of the present specification, wherein:

FiGURE 1 is a diagrammatic illustration of the singlecabinet speaker array utilized according to these teachings, shown in connection with a circuit diagram of the means for energizing the several speaker systems therein; and

FIGURE 2 is a diagrammatic indication of the sound levels of the several speakers of the group, as related to the total sound output of the combined speaker systems.

As indicated heretofore, the present sound system is so designed as to be compatible with conventional existing stereophonic amplifying apparatus, which conventionally comprises a pair of amplifiers 10 and 11 generally identical, but usually designated as the left channel amplifier and right channel amplifier, respectively. In common practice such amplifiers usually utilize a push-pull output stage, including paired output tubes 12 and 13, respectively, with their plates 14 and 15 connected to the opposite ends of center-tapped primary windings 16 and 17 of paired output transformers generally designated 13 and 19, respectively.

The amplifiers 10 and 11 are arranged to be energized by conventional double-channel stereophonic signals connected from a source (such as a stereo cartridge or plural head tape recorder) through connections 21-22 and 23-24, respectively, but it is to be noted that while these signals are carried directly to the input terminals 25 and 26 of the left amplifier, for example, the connections of the other amplifier are reversed so that opposite polarity is applied to the input terminals 25 and 26 of the right channelamplifier 11. Obviously, this reversal of polarity follows through the secondary windings, generally designated as 27 and 28, respectively, so that although the windings are shown in reverse orientation with respect to each other, the instantaneous polarity will be as indicated by the positive and negative signs thereon.

The windings 27 and 28 may be identical, but are specially constructed to achieve the purpose of this disclosure and to apply the signal induced therein to the several speakers of the sound system in the manner to achieve the desired stereophonic effect. To this end, the windings 27 and 28 of the transformers 1S and 19 are each made up of three sections in series with each other, with a turns ratio designed to maintain a predetermined and constant ratio of output potential between the sections 2'7A, 27B and 27C. The same arrangement exists between the sections 28A, 28B and 28C of thewinding 28. In practice, the windings 2'7 and 28 have an over-all impedence of 13 ohms, with a turns ratio of 5-8-3 for the sections A, B and C. They are so designed that, when operating at a 20 watt level, section A of each transformer produces an output potential of about 5 volts, section B 8 volts, and section C 3 volts, for a total of about 16 volts across the entire winding.

The winding 27 serves as the principal means of energizing the left-hand speaker system, while winding 28 is the primary source of energy for the right speaker system. Each of these speaker systems may comprise one or more individual speakers, in coacting relation. As shown, however, the left system comprises the single speaker 31 mounted in the left Wall of the speaker cabinet 29. Speaker 31 is arranged on a horizontal axis, and outwardly faced from the cabinet to project sound to the left in the general direction of the arrow 32. To this end, one terminal of the transformer Winding 27 extends through the lead 33 to a 35 mfd. capacitor 34 from which a lead 35 extends directly to one terminal of speaker 31. The opposite terminal of the speaker and choke are connected through a lead 37 to the end of section 28C of the winding 28. The speaker is paralleled by a 7 mh. choke coil 36 which coacts with capacitor 34 to form a speaker dividing network, effective to roll oif reproduction below 250 cycles per second, as shown in FIGURE 2. The circuit is completed by a jumper 38 extending from the junction of the windings 23B and 28C to the junction of windings 27B and 27C. As illustrated, this jumper is preferably grounded at 39.

The right-hand speaker system is shown as comprising the single speaker 41 disposed on a horizontal axis within the cabinet 32 and arranged to project sound to the right in the direction of the axial arrow 42. This speaker is connected with the output Winding 23 through lead 43 to 35 mfd. capacitor 44 and lead 45, and is bridged by 7 mh. choke as, which coacts with the capacitor to form a speaker divider network in the same manner as capacitor 36 and choke 38. Similarly, the circuit from speaker 41 is completed through crossover lead 47 which extends to the end of windin section 270 and thence through jumper 35 back to the right channel output transformer.

The front, or forwardly directed speaker system may comprise one or more speakers, but, as shown, this system consists of a non-directional low frequency woofer 51 and a high frequency tweeter '2 having more well-defined directional characteristics, both mounted on horizontal axes in the forward wall of the speaker cabinet 2? and forwardly directed as indicated by the arrow 53, it being understood that while these speakers may be coaxially disposed, if desired, they are preferably separate units mounted in any convenient spaced relation with each other on the front wall of the cabinet. The front speaker system 51-52 is energized through the winding sections 27B and 28B of the output transformers I8 and 19 by leads 54 and 55, with a 4.7 ohm resistor es and 2.5 mfd. capacitor 57 in series with the tweeter 52 and with a 47 ohm resistor 58 and 5 mh. choke 5? parallel with each other and in series with the woofer 51 to form a speaker dividing network having a response curve as shown by lines 65-67 of FIGURE 2, in the manner well known in the art.

It will be observed from FIGURE 1 that while the windings of the transformers 18-19 are shown in reversed (mirror-image) relation to each other, the reversal of polarity accomplished by the cross-over of leads 23-24 results in corresponding polarity reversal of the secondary windings and resultant opposite instantaneous polarity of leads 33 and 43, as indicated. The speaker 31 will thus be primarily energized by winding sections 27A and 273 in series, opposed by the lower potential winding section 23C. Similarly, speaker 41 will be primarily energized by sections 28A and 283 in series, opposed by 27C. The resultant effect is that speakers 31 and 41 are normally 180 degrees out of phase (as modified by the differences in signal information between the left and the right channels) so that, with identical signals, the speaker cones move in unison and in the same direction, as indicated by the arrows 61, 62. The voltage ratio .231 provided by the potential of winding 23C divided by the series potential of windings 27A plus B has been found to give a most satisfactory stereo effect.

The ratio of transformer windings indicated above, particularly with speaker divider networks of the values indicated, results in operation or" the several speakers of the system as indicated in the graphic representation in FIGURE 2, wherein line 63 represents the total volume of all speakers. In this diagram, lines 64 and 65 representing both side speakers and one side speaker respec tively, showing the low frequency roll-off at about 250 cycles per second, the line 66 represents the volume of the woofer 51 which drops about 6 decibels above 250 cycles per second, while line 67 represents the tweeter 52 which crosses over the woofer at 2500 cycles.

The combined result of operation according to the present disclosure is a lifelike stereophonic effect, accomplished over a relatively broad listening area and characterized by realistic illusion as to directional characteristics of the reproduced sound. This is accomplished, moreover, by apparatus capable of true tonal quality throughout the operating range of the system and designed to utilize dual channel audio amplifiers of conventional design. The stereo effect is thus achieved in a manner whereby the entire sound reproduction unit may be housed within a single cabinet no larger than required to mount the speakers, without need for dual cabinets or remote speaker units.

Having thus described the invention, what I claim as new and desire to protect by United States Letters Patent is:

1. In a stereophonic sound system, paired output trans1 formers with the secondaries of each of said output transformers tapped to comprise plural windings in series; a speaker array housed within a single cabinet and including at least two side speakers and at least one front speaker, all closely grouped with respect to each other, with each of said side speakers energized by a major portion of the secondary winding of one of said output transformers in series and opposed phase relation with a minor portion of the secondary winding of the other transformer; with a circuit through a portion of the secondary winding of each of the transformers in series and in coacting phase relation energizing the aforesaid front speaker.

2. In a stereophonic sound system, paired output transformers with the secondaries of each of said output transformers tapped to comprise plural windings in series; a speaker array housed within a single cabinet and including at least two side speakers and at least one front speaker, all closely grouped with respect to each other, with each of said side speakers energized by a major portion of the secondary Winding of one of said output transformers in series and opposed phase relation with a minor portion of the secondary winding of the other transformer.

3. In a stereophonic sound system including paired amplifiers energizing the primary windings of a pair of output transformers, a sound reproduction system comprising a pair of loud speakers closely spaced with respect to each other and facing away from each other, with each of said speakers energized by a major portion of the output winding of one of said output transformers in series with and in opposed phase relation with a minor portion of the output winding of the other output transformer, and with the speakers in opposite phase relation to each other with respect to the major signal transmitted thereby.

4. In a stereophonic sound system including first and second amplifiers, an output signal from each said amplifier; first means for deriving major and minor portions of the output signal from said first amplifier; second means for deriving major and minor portions or" the output signal from said second amplifier; a sound reproduction apparatus comprising a first and a second loud speaker closely spaced with respect to one another within a housing and facing away from one another; circuit means conductively connected to energize said first speaker with the major portion of the signal derived from said first amplifier in series and in opposed phase relationship with the minor portion of the signal derived from said second amplifier, said circuit means further conductively connected to energize said second speaker with the major portion of the signal derived from said second amplifier in series and opposed phase relationship with the minor portion of the signal derived from said first amplifier; and said circuit means further connecting said first and said second loud speakers in mutually opposed phase relation with respect to the major signal portions.

5. A stereophonic sound system as claimed in claim 4, further including an intermediate loud speaker system disposed at right angles to said first and second loud speakers, and circuit means conductively connecting the intermediate loud speaker system for energization by a portion of the signal derived by both said first and said second deriving means.

References Cited in the file of this patent UNITED STATES PATENTS Waite Jan. 10, 1939 De Boer Sept. 16, 1952 OTHER REFERENCES 6 High Fidelity and Audiocraft Magazines; McGraw-Hill Book Co., Inc., New York; copyright 1956; pp. 287-290.

Augspurger: New Ideas In Stereo Speaker System; Radio-Electronics, Mar. 1959, pp. 64-67.

Burstein: Stereo Amplifier Controls; Electronics World; August, 1959; pages 55-57, 122.

Shottenfield: The Stereo Plus System; Audio, Oct. 1959, pages 23, 114-116.

Klipsch: Circuits For Three-Channel Stereophonic Playback Derived From Two Sound Tracks; IRE Transactions On Audio, Nov.-Dec. 1959, pp. 161-165.

Extended Stereo System; Electronics World, Jan. 1960, pp. 50,156, 157.

Burstein: Add a Third Speaker the Easy Way; Radio- Fowler: High Fidelity; A Practical Guide; Publisher, 15 Electronics, 1960, P 

4. IN A STEREOPHONIC SOUND SYSTEM INCLUDING FIRST AND SECOND AMPLIFIERS, AN OUTPUT SIGNAL FROM EACH SAID AMPLIFIER; FIRST MEANS FOR DERIVING MAJOR AND MINOR PORTIONS OF THE OUTPUT SIGNAL FROM SAID FIRST AMPLIFIER; SECOND MEANS FOR DERIVING MAJOR AND MINOR PORTIONS OF THE OUTPUT SIGNAL FROM SAID SECOND AMPLIFIER; A SOUND REPRODUCTION APPARATUS COMPRISING A FIRST AND A SECOND LOUD SPEAKER CLOSELY SPACED WITH RESPECT TO ONE ANOTHER WITHIN A HOUSING AND FACING AWAY FROM ONE ANOTHER; CIRCUIT MEANS CONDUCTIVELY CONNECTED TO ENERGIZE SAID FIRST SPEAKER WITH THE MAJOR PORTION OF THE SIGNAL DERIVED FROM SAID FIRST AMPLIFIER IN SERIES AND IN OPPOSED PHASE RELATIONSHIP WITH THE MINOR PORTION OF THE SIGNAL DERIVED FROM SAID SECOND AMPLIFIER, SAID CIRCUIT MEANS FURTHER CONDUCTIVELY CONNECTED TO ENERGIZE SAID SECOND SPEAKER WITH THE MAJOR PORTION OF THE SIGNAL DERIVED FROM SAID SECOND AMPLIFIER IN SERIES AND OPPOSED PHASE RELATIONSHIP WITH THE MINOR PORTION OF THE SIGNAL DERIVED FROM SAID FIRST AMPLIFIER; AND SAID CIRCUIT MEANS FURTHER CONNECTING SAID FIRST AND SAID SECOND LOUD SPEAKERS IN MUTUALLY OPPOSED PHASE RELATION WITH RESPECT TO THE MAJOR SIGNAL PORTIONS. 